Concomitant with increased interest and importance in marine resources exploration, marine environmental surveillance and underwater military defenses, demand on underwater communication capable of collecting multifarious underwater information from oceans has recently increased. The underwater communication is performed using ultrasonic waves because of physical properties of media. A communication network for underwater information transmission may be implemented by installing a sensor node capable of performing transmission/reception of underwater information under underwater environments, and by obtaining and controlling underwater information from the sensor node.
Because of underwater communication environments using ultrasonic waves, the underwater communication network is relatively smaller in bandwidth of signal that is transmitted than the land communication, and signal attenuation relative to distance is also very large. That is, frequencies used in underwater communication network have to be very limited in order to perform a reliable communication at a distance of several kilo meters to several scores of kilo meters.
Moreover, when the demand on underwater information obtainment using underwater communication network increases, the number of sensor nodes performing communication in the underwater also increases. However, the conventional underwater communication network failed to efficiently control the sensor nodes due to limitation of frequencies useable at the underwater channel environments. That is, when only one frequency is used to perform the communication in the conventional underwater communication network, and when a relevant frequency is allocated to one sensor node, all other sensor nodes cannot transmit or receive a signal.
Furthermore, when communication is performed using a plurality of frequencies in the conventional underwater communication network, and when the number of sensor nodes wanting to perform a communication in the water is greater than the allocated frequency, the underwater sensor nodes as many as the number exceeding the allocated frequency cannot transmit or receive a signal. In addition, in this case, all sensor nodes must continuously inspect what frequency an ambient sensor node uses by being allocated, such that battery consumption greatly increases in the water to greatly decrease an operation period of underwater sensor node.
Thus, the conventional underwater communication network has limited the number of communicable sensor nodes because a plurality of sensor nodes cannot be efficiently managed. On top of that, it has become inevitable to increase the number of sensor nodes due to increases in various demands on marine information, and therefore, the trend is that an efficient control of underwater communication network in various areas is greatly required.